[GPU] Add texture signedness enumeration

For sign_x, sign_y, sign_z and sign_w fields of xe_gpu_texture_fetch_t.

src/xenia/gpu/xenos.h

@@ -63,6 +63,17 @@ enum class ClampMode : uint32_t {
   kMirrorClampToBorder = 7,
 };
 
+// TEX_FORMAT_COMP, known as GPUSIGN on the Xbox 360.
+enum class TextureSign : uint32_t {
+  kUnsigned = 0,
+  // Two's complement texture data.
+  kSigned = 1,
+  // 2*color-1 - http://xboxforums.create.msdn.com/forums/t/107374.aspx
+  kUnsignedBiased = 2,
+  // Linearized when sampled.
+  kGamma = 3,
+};
+
 enum class TextureFilter : uint32_t {
   kPoint = 0,
   kLinear = 1,